THE DAKOTAN TECTONIC ZONE RECORDS FINAL SUTURING OF WYOMING AND SUPERIOR PROVINCES AT 1750–1715 MA; NORTHEASTERN BLACK HILLS, SD
ENE-WSW shortening during D2 deformed the < 2.4 Ga supracrustal rocks into upright F2 folds with near-horizontal, NNW-striking axes. Prior to this research, D3 structures were limited to rare F3 folds with upright, NW-striking axial planes and near-vertically plunging axes. Our studies support a strong partitioning of D3 strain along the eastern margin of the Black Hills. Within this high-strain zone, F3 folds are pervasive and have a strong spatial and kinematic correlation with similarly oriented ductile shear zones. We propose these D3structures combine to form the Dakotan Tectonic Belt recording left-lateral, NE-up oblique slip during NW-SE shortening,
In the NE Black Hills, D2 folds are not recognized in the >2.4 Ga rocks of the Nemo Group, nor in the Archean basement rocks in the Little Elk terrane; however, D3 structures are abundant into both these older lithologies. Additionally, whereas D2 structures are limited to the Black Hills, structures with ages and shortening directions similar to D3 are present to the SW in the Hartville uplift and the Laramie Mountains. The involvement of Archean basement rocks, and the greater regional extent of this event support the interpretation that D3 represents the final continent-continent suturing of the Wyoming and Superior provinces during the Dakotan Orogeny.